In recent years, plant growth using an artificial light source has been studied. In particular, attention is being paid to a cultivation method using an illumination by a light emitting diode (LED) which has excellent monochromaticity and in which energy saving, long service life, and a reduction in size are possible.
From previous study results, as one luminescence wavelength suitable for a light source for plant growth (photosynthesis), the effects of blue light having a wavelength around 450 nm and red light having a wavelength range of 600 nm to 700 nm have been confirmed. In particular, with respect to photosynthesis, light having a wavelength of around 660 nm to 670 nm is the most desirable light source having high reaction efficiency.
In plant cultivation, a photosynthetically effective photon flux density is used as light intensity. The photosynthetically effective photon flux density represents the number of photons per unit time and area, of light in a visible light region effective for photosynthesis. Red and blue light is light in a visible light region effective for photosynthesis.
The light intensity of a light source for plant growth is evaluated by the number of photons, namely the photon flux (μmol/s). Moreover, the light intensity of the irradiation surface corresponding to the performance of an illumination apparatus is evaluated by the photon flux density (μmol/s·m2) which is the incident photon flux per unit area of the irradiation surface.
Moreover, the balance in the light intensities of red and blue light is known to be an important factor to the growth of plants. Specifically, it is preferable that for many plants, though it depends on the kind of plant, red and blue photons are balanced such that red photons are several times (for example, 2 to 10 times) stronger than blue photons.
In the conventional light emitting diode lamp, in order to irradiate plants with blue and red light with an equal intensity ratio, a method of disposing a plurality of red lamps and a plurality of blue lamps in a mixed manner, a method of controlling light distribution, the production of a light emitting diode including a light emitting layer that emits red and blue light, and the like have been studied (for example, see PTLs 1 to 3).
However, although AlxGa1-xAs is used in the light emitting layer of the conventional red LED, since the luminescence efficiency of the red LED is lower than that of the blue LED, it is desirable to improve the luminescence efficiency of the red LED. Moreover, when using a red LED having low luminescence efficiency, in order to obtain a desirable mixed color light ideal for plant growth, a number of red LEDs are required for one blue LED. Thus, since the number of red lamps is different from the number of blue lamps, a large number of red LEDs are interspersed around the blue LEDs, and it is difficult to irradiate the mixed color light uniformly. Moreover, in order to irradiate the mixed color light uniformly, it is necessary to light all the LEDs in units of blocks in which the blue LED is used as a control factor.
In contrast, as a LED having high luminescence efficiency, a LED including a light emitting layer made of aluminum gallium indium phosphide (composition formula: (AlXGa1-X)YIn1-YP (0≦X≦1 and 0<Y≦1)) is known. However, in the LED including the light emitting layer, a light emitting layer having a composition of Ga0.5In0.5P has the longest wavelength, and its peak wavelength is around 650 nm. In a wavelength region longer than 655 nm, it is difficult to attain practical realization and obtain a high output. Thus, there is a problem in that the light emitting layer having the light emitting layer is not ideal for a light source for plant growth.
[PTL 1] Japanese Unexamined Patent Application Publication No. 8-103167
[PTL 2] Japanese Unexamined Patent Application Publication No. 2001-86860
[PTL 3] Japanese Unexamined Patent Application Publication No. 2002-27831